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A single-electron transistor scanning electrometer (SETSE)-a scanned probe microscope capable of mapping static electric fields and charges with 100-nanometer spatial resolution and a charge sensitivity of a small fraction of an electron-has been developed. The active sensing element of the SETSE, a single-electron transistor fabricated at the end of a sharp glass tip, is scanned in close proximity across the sample surface. Images of the surface electric fields of a GaAs/AI^sub x^Ga^sub 1^ -^sub x^As heterostructure sample show individual photo-ionized charge sites and fluctuations in the dopant and surface-charge distribution on a length scale of 100 nanometers. The SETSE has been used to image and measure depleted regions, local capacitance, band bending, and contact potentials at submicrometer length scales on the surface of this semiconductor sample.
Inspired by the development of the scanning tunneling microscope, a variety of surface scanning probes (1) have been developed to measure and map properties of material surfaces on a microscopic scale. In particular, surface electrical properties have been explored with noncontact techniques such as scanning capacitance microscopy (2), scanning Kelvin probe microscopy (3), and electric-field-sensitive atomic force microscopy (EFM) (4). Indeed, the last has in one instance (5) shown the remarkable ability to detect the presence of individual charges and to obtain images of insulating surfaces in which a charged spot of one or two electrons is apparent.
We report the development of a lowtemperature scanning electrometer operating on a different principle, one which has one to two orders of magnitude greater charge resolution and a similar spatial resolution (100 nm) compared with the EFM. This microscope, the single-electron transistor (SET) scanning electrometer, or SETSE, uses the SET as a probe to sense the electrically induced charge on its small (100 nm) metal island held in proximity to the sample surface (Fig. IC). It can detect -1% of an electron charge (0.01e). Because all of the important geometrical parameters are known, one can assign a quantitative interpretation to the SETSE signal. Also, during operation the SETSE, unlike the EFM, does not require the application of high electric fields (106 V cm-l) between the tip and surface, an important consideration for many interesting but easily perturbed semiconductor systems. All of these features enable a broader class of experiments to...